Stable, viewable black and white photographs can be produced by imagewise exposing a photographic element containing one or more radiation sensitive silver halide emulsion layers capable of producing a developable latent image. To extend the response of the silver halide into the green and/or red regions of the visible spectrum and thereby better approximate the image seen by the human eye it is common practice to adsorb a spectral sensitizing dye to the surfaces of the silver halide grains in the emulsion layers. Following imagewise exposure a viewable image can be produced by development in an aqueous alkaline processing solution. The imagewise conversion of silver halide to metallic silver provides the viewable image. To avoid an eventual increase in density attributable to residual silver halide it is common practice to fix out (dissolve and remove by washing) the residual, undeveloped silver halide grains. This leaves a stable, viewable silver image in the photographic element.
In silver halide photography a choice of three halides, chloride, bromide, and iodide, and combinations thereof are available. Silver iodide is known to be the most difficult silver halide to employ for producing a latent image and developing and is seldom used alone in emulsions intended to be processed by development in aqueous alkaline solutions followed by fixing out. When present in a photographic element silver iodide is often relegated to performing functions which do not require the formation of a developable latent image in silver iodide grains. The following are illustrative of known uses of silver iodide grains and soluble iodide salts:
P-1 Carroll U.S. Pat. No. 2,327,764 discloses the use of silver iodide as an ultraviolet filter for a color photographic element; PA0 P-2 Van Pee et al. U.S. Pat. No. 3,745,015 discloses the incorporation of a silver iodide sol in a direct print radiation sensitive silver halide emulsion; PA0 P-3 Maskasky U.S. Pat. No. 4,094,684 discloses radiation sensitive silver iodide grains onto which have been epitaxially grown silver chloride; PA0 P-4 Maternaghan U.S. Pat. No. 4,184,878 discloses the use of high iodide silver halide grains as seed grains in preparing tabular grain silver bromoiodide emulsions; PA0 P-5 U.K. Specification No. 1,413,826 discloses the use of 0.01 to 1.0 mole percent soluble iodide to assist in the spectral sensitization of silver bromoiodide; PA0 P-6 Maskasky U.K. Specification No. 2,132,373 discloses gamma phase tabular grain silver iodide emulsions; and PA0 P-7 Japanese Kokai Pat. No. Sho 52[1977]-130639 discloses the use of potassium iodide in a fixing solution to increase fixing speed. PA0 P-8 Mignot U.S. Pat. No. 4,386,156 discloses a tabular grain silver bromide emulsion wherein tabular silver bromide grains bounded by {100} major crystal faces and having an average aspect ratio of at least 8.5:1, account for at least 50 percent of the total projected area of the silver bromide grains present in the emulsion; PA0 P-9 Wey U.S. Pat. No. 4,399,215 discloses a tabular grain silver chloride emulsion wherein the tabular grains have an average aspect ratio greater than 8:1; PA0 P-10 Maskasky U.S. Pat. No. 4,400,463 discloses a tabular grain emulsion the grains of which are at least 50 mole percent chloride and have one or more edges of a particular crystallographic orientation; PA0 P-11 Dickerson U.S. Pat. No. 4,414,304 discloses fully forehardened photographic elements capable of producing a stable, viewable silver image of increased covering power by reason of containing a high aspect ratio tabular grain silver halide emulsion; PA0 P-12 Wey et al. U.S. Pat. No. 4,414,306 discloses tabular grain silver halide emulsions wherein the halide is a combination of chloride and bromide; PA0 P-13 and P-14 Abbott et al. U.S. Pat. Nos. 4,425,425 and 4,425,426 disclose radiographic elements containing silver halide emulsion layers on opposite major faces of a support. High and intermediate aspect ratio tabular grain silver bromide emulsions are specifically disclosed; PA0 P-15 Maskasky U.S. Pat. No. 4,435,501 discloses the selective site epitaxial sensitization of high aspect ratio tabular grain silver halide emulsions; PA0 P-16 Kofron et al. U.S. Pat. No. 4,439,520 discloses efficiently chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions; and PA0 P-17 Daubendiek et al. U.K. Specification No. 2,110,831A discloses direct positive silver halide emulsions containing internal latent image forming high aspect ratio tabular grain emulsions.
The highest speed silver halide emulsions are silver bromoiodide emulsions, which are most frequently employed for camera speed imaging. These emulsions contain bromide as the predominant halide. Silver iodide can be present up to its solubility limit in silver bromide, about 40 mole percent, but is seldom employed in concentrations above 20 mole percent and is usually employed in concentrations below 10 mole percent.
For a number of photographic applications processing speed and convenience are of paramount importance. Silver chloride, silver bromide, and silver chlorobromide emulsions are outstandingly suited for these applications, since they can be more rapidly processed than silver iodide or silver bromoiodide emulsions. Further, acceptable processing of these emulsions can be obtained with greater variances in the time and temperature of processing.
Interest in silver halide photography has recently focused on tabular grain emulsions, particularly intermediate and high aspect ratio tabular grain emulsions. It has been shown that the latter emulsions can produce increased image sharpness. When efficiently chemically and spectrally sensitized, these emulsions exhibit outstanding speed-granularity relationships. Higher silver covering power has been observed in fully forehardened photographic elements. In radiographic elements with emulsion coatings on each of the two opposite faces of the support marked reductions in crossover have been observed using high aspect ratio tabular grain emulsions, and improvements in speed at comparable crossover levels have been demonstrated using thin, intermediate aspect ratio tabular grain emulsions.
Photographic elements containing tabular grain silver bromide, silver chloride, and silver chlorobromide emulsions as well as their sensitization, use, and advantages are illustrated by the following:
A disadvantage that has been discovered with the use of spectrally sensitized tabular grain silver bromide, silver chloride, and silver chlorobromide emulsions in producing stable, viewable silver images is dye stain. In contrast to spectrally sensitized silver halide emulsions of similar halide content which are not tabular grain emulsions, sufficient residual spectral sensitizing dye remains in the photographic element at the conclusion of processing to increase the density in the low and intermediate density regions of the image bearing photographic element. Dye stain can be undesirable in altering image tone. Variations in image tone are particularly undesirable in radiography, since this can complicate proper interpretation of X-ray images. Further, residual dye stain is objectionable in that it does not affect all wavelengths equally. Rather, it is particularly large at wavelengths at or near the absorption peak of the dye. Residual dye stain is highly objectionable where it is desired to scan the photographic image with a laser of a wavelength approximating the absorption peak of the spectral sensitizing dye.